As summer 2026 approaches, you spend a day at the beach. You meant to reapply sunscreen after swimming but got distracted. By evening, your skin is tight and red. By the next morning, it is painful to touch, and a few days later it starts peeling. Most people think of sunburn as a surface-level problem, a result of too much heat, too much light, and your skin just gets irritated. The reality is far more interesting and considerably more alarming. Sunburn is your body's response to molecular damage happening inside the nucleus of your skin cells. The redness, the pain, and the peeling is the result of ultraviolet radiation chemically altering your DNA.
The UV Spectrum
Ultraviolet radiation sits just beyond the visible spectrum, at wavelengths shorter than violet light. It is divided into three bands, each with having different biological effects.
UVC (100–280nm) is the most energetic and potentially the most damaging, but it is almost entirely absorbed by the ozone layer and does not reach the Earth's surface under normal conditions. UVB (280–315nm) is the primary driver of acute DNA damage and sunburn. It is high energy, directly absorbed by DNA bases, and responsible for most of the molecular disruption. UVA (315–400nm) is lower energy but present in far greater quantities, and accounts for around 95% of the UV radiation that reaches the Earth's surface. UVA penetrates deeper into the skin, contributes to photoageing, and causes indirect DNA damage through the generation of reactive oxygen species. Crucially, UVA is not blocked by most windows and is present at relatively constant intensity throughout the day and year.
Tanning beds emit predominantly UVA radiation, which is why they were long marketed as a safer alternative to sun exposure. That assumption turned out to be wrong as UVA causes significant DNA damage through indirect pathways and is strongly linked to melanoma risk. There is no biologically safe form of UV-induced tanning.
What UV Does to DNA
DNA is made of four bases: adenine, guanine, cytosine, and thymine. Cytosine and thymine are pyrimidines, a structure which makes them particularly effective at absorbing UVB photons, which is where the damage begins.
When a UVB photon is absorbed by two adjacent thymine bases sitting on the same DNA strand, it provides enough energy to drive the formation of a covalent bond between the two bases. The result is a thymine dimer, more formally called a cyclobutane pyrimidine dimer, or CPD. Cytosine-thymine and cytosine-cytosine dimers can also form, though thymine-thymine dimers are the most common. A second class of lesion (6-4 photoproducts) involves a different covalent bond between adjacent pyrimidines that distorts the double helix even more severely.
The consequences of dimer formation are significant as the two bases are now covalently bonded together, physically buckling the DNA strand and preventing normal base pairing with the complementary strand. As a result, DNA replication stalls as the polymerase enzyme that copies DNA during cell division cannot read through the dimer and stops. Secondly, transcription is disrupted as the RNA polymerase that reads DNA to produce messenger RNA cannot pass through the damaged site, shutting down protein production from that region of the genome. On a moderately sunny day, unprotected skin cells can accumulate tens of thousands of pyrimidine dimers per hour.
The Body's Response
The visible symptoms of sunburn are not caused directly by UV radiation, but is caused by the cellular response to DNA damage.
Cells that detect significant DNA damage release a cascade of inflammatory signals. Among these are prostaglandins and cytokines, which together trigger vasodilation and increased blood flow to the affected area, producing the characteristic redness and heat of sunburn. Pain receptors in the skin are sensitised by these same inflammatory mediators, which is why sunburned skin hurts to touch.
Simultaneously, a protein called p53 is activated in response to DNA damage. p53 is a tumour suppressor, and its job is to prevent cells with damaged DNA from dividing and potentially passing mutations to daughter cells. It does this by triggering either cell cycle arrest (the pausing of replication to give the cell time to repair) or apoptosis, programmed cell death, if the damage is too severe to be fixed.
The peeling that follows sunburn is the visible consequence of mass apoptosis. The body is systematically eliminating skin cells whose DNA has been too severely damaged to safely replicate, acting as a protective mechanism. By destroying heavily damaged cells before they can divide, the body reduces the risk that a cell carrying unrepaired mutations will go on to become cancerous.
DNA Repair
The human body has not simply evolved to destroy UV-damaged cells, it has also evolved sophisticated machinery to repair them. The primary pathway for UV-induced DNA damage is nucleotide excision repair (NER).
NER proceeds through five stages. First, recognition means that specialised damage-sensing proteins detect the distortion in the double helix caused by a pyrimidine dimer. The physical buckle in the strand is the signal, not the chemical identity of the lesion, which is why NER can recognise a wide variety of structurally distorting damage. Secondly, the unwinding of the structure, as helicase enzymes unwind the double helix around the damage site, exposes the affected region. Third, excision, as endonuclease enzymes cut the damaged strand on both sides of the lesion, removing a short single-stranded fragment of around 25–30 nucleotides containing the dimer. Fourth, resynthesis occurs as DNA polymerase fills in the gap using the intact complementary strand as a template, restoring the correct sequence. Finally, ligation, as DNA ligase seals the backbone, restoring the continuous double helix.
When Repair Fails
NER is accurate and efficient, but it is not unlimited. Under heavy or repeated UV exposure, the volume of damage can outpace the repair machinery. When a pyrimidine dimer is not repaired before the cell enters replication, the DNA polymerase faces a problem as it cannot read through the dimer normally. In some cases, a specialised process called translesion synthesis allows replication to continue past the lesion, but the translesion polymerases that perform this bypass are error-prone, and they frequently insert an incorrect base opposite the damaged site.
If these mutations accumulate in critical genes, particularly tumour suppressors like p53 itself, they can initiate the cellular transformation that leads to skin cancer. The three main forms are basal cell carcinoma and squamous cell carcinoma, both strongly associated with overall lifetime UV exposure, and melanoma, the most dangerous form, linked to intense intermittent exposure and a history of sunburn. Melanoma accounts for only around 1% of skin cancer cases but is responsible for the majority of skin cancer deaths, a disparity explained by its capacity to metastasise aggressively if not caught early.
How Sunscreen Actually Works
Chemical sunscreens contain organic molecules: including avobenzone, oxybenzone, and octocrylene, with conjugated ring systems that absorb UV photons and dissipate the energy as heat through molecular vibration before it can reach DNA. These molecules are essentially sacrificial UV absorbers as they take the photon hit so your DNA does not have to. Physical sunscreens contain inorganic compounds including zinc oxide and titanium dioxide that physically scatter and reflect UV photons at the skin surface. Physical sunscreens tend to offer broader spectrum coverage and are less likely to cause skin irritation, which is why they are often recommended for sensitive skin and young children.
SPF is frequently misunderstood. A common misconception is that SPF 30 blocks 30 times more UV than SPF 1. It instead means that it takes 30 times longer for UVB radiation to produce the same degree of redness as unprotected skin. In practical terms, SPF 30 blocks around 97% of UVB radiation, while SPF 50 blocks around 98%. The difference between SPF 30 and SPF 50 is much smaller than most people assume, but both are meaningfully better than nothing.
Two further points are worth noting. First, most SPF ratings measure UVB protection only, and in the future you should look for broad spectrum labelling to ensure UVA coverage as well. Second, chemical sunscreens undergo photodegradation, which means that the organic molecules are consumed as they absorb photons, gradually losing their effectiveness, which means that you should reapply every two hours.
Emily Jong